Equipment and machinery is often capable of injuring or killing a person when it is operated unsafely. For example, a microwave oven can cause death or injury if it is operated with the door open. The open microwave oven door is an unsafe condition. Safety interlock circuits are electric circuits designed to prevent equipment and machinery from operating when an unsafe condition exists. FIG. 1, labeled as “prior art”, illustrates a basic safety interlock circuit. In order to function, the circuit must have a signal to carry. The input signal is introduced into the circuit through the circuit input terminal 101. The signal then passes through three safety interlock switches 102 and into a machine input terminal 104. Finally, the signal exits the machine 105 at the machine output terminal 106 and exits the safety interlock circuit at the circuit output terminal 107. The machine 105 only operates if the signal can pass through it. Any opening in the circuit will prevent the machine 105 from operating. A safety interlock switch 102 must be in the closed state for the signal to pass through it. In the safety interlock circuit of FIG. 1, all the safety interlock switches 102 must be closed or the machine 105 will not operate.
Returning to the microwave oven example, the circuit input terminal 101 and circuit output terminal 107 could be the prongs on the power cord that is plugged into the wall. In that case, the signal is the AC line power used to power the microwave oven. One safety interlock switch 102 is the door safety switch that opens whenever the door opens. Another safety interlock switch 102 can be set to open whenever the top cover of the microwave oven is removed. The machine 105 is all the parts that rotate food or generate microwave radiation. In this example, the microwave oven cannot operate with either the door open or top cover removed because a safety interlock switch cuts the AC line power.
Some equipment requires more power than can be safely carried in a safety interlock circuit. In this case, a control module 202 is required as shown in FIG. 2, which is labeled as “prior art”. In FIG. 2, the signal passes into the control input terminal 201, through the control module 202, and out the control output terminal 203. Electric power for the machine 105 passes into the power input terminal 204, through the control module 202, through the machine 105, and out the power output terminal 106. The control module 202 switches machine power on and off based on the presence or absence of the signal.
Electric relays are often used for control modules. An electric relay is a common electrical component that uses an electric current as a control signal for opening and closing a switch. Those skilled in the art of electric circuitry know the properties of relays and many functional equivalents of relays wherein a control signal switches power on and off. Some of the similar devices are transistor, vacuum tubes, silicon controlled rectifiers and field effect transistors.
A significant problem with safety interlock circuits is that it is often impossible to know which particular safety interlock switch is disabling the machinery. In the microwave oven example, it is easy to see if the door is open. However, if the machinery is an elevator in a skyscraper, the open switch could be on any floor of the building. If a safety interlock switch has disabled an elevator, then the time spent by the maintenance crew just to isolate the problem can be considerable. There are safety interlock switches that can report their state, but they also require a dedicated signaling circuit. A safety interlock circuit with such switches is shown in FIG. 3, labeled as “prior art”, wherein each switch uses a dedicated signal wire 302 operable through a monitoring module 301. Another safety interlock circuit is shown in FIG. 4, labeled as “prior art”, wherein the switches share a common signal bus 401.
Examples of a monitoring module 301 are devices that actively monitor interlock switch position, interlock switch contacts, voltage across the interlock switch contacts, or current through the interlock switch. Certain types of switches can monitor their own switch position because they independently open or close multiple independent circuits.
The solutions of FIG. 3 and FIG. 4 are both used, but they both require signal wires to be installed. There are many installations that already have a safety interlock circuit installed that would benefit from a reporting mechanism for safety interlock switches. However, the wires are already installed and it is often prohibitive to install new wires because some machines are literally miles long.
Current diagnostic solutions for safety interlock circuits require additional wiring and additional switches or switch contacts as well as all the extra time, support circuitry, and expense involved in installing, using and maintaining them.
The present invention directly addresses the shortcomings of the prior art by supplying a signaling mechanism that does not require more wires, switches, or switch contacts than any presently installed safety interlock circuit.